The pressure intermediary of the invention includes: A platform, with a membrane- or diaphragm-bed on a surface of the platform; a separating membrane, or diaphragm, which is secured to the platform in its edge region to enclose a cavity; and a hydraulic path, which communicates with the cavity through an opening in the surface of the platform; wherein the cavity and the hydraulic path are filled with a pressure-transfer liquid.
Pressure intermediaries of this type are basically known, and the state of the art includes a multitude of embodiments of separating membranes, which are optimized for the most varied of conditions.
For example, DE 100 31 120 A1 discloses a pressure intermediary proposed for minimizing or eliminating a temperature-dependent membrane error. This temperature-dependent error stems from elastic deformation of the separating membrane due to pressure in the pressure-transfer liquid resulting from the temperature-dependent volume change of the pressure-transfer liquid. DE 100 31 120 A1 discloses, for example, a pressure intermediary having a bowl-shaped separating membrane, which has a flat edge region for attachment of the separating membrane to a platform and a lightly waved, central region depressed relative to the edge region. Extending between the edge region and the central region is an inclined transition region connecting the edge and central regions together. The separating membrane and the platform are matched to one another in such a manner that the equilibrium position of the separating membrane changes as a function of temperature, and, in fact, in such a manner that the resulting volume in the cavity between the separating membrane and the platform corresponds to the temperature-dependent volume of the pressure-transfer liquid. This is achieved by, among other things, providing the platform with a greater coefficient of thermal expansion than the separating membrane. Although the described state of the art according to DE 100 31 120 A1 might be of interest for thermal equilibrium situations, nevertheless in the case of temperature differences between the platform and the separating membrane, which can arise, for example, following CIP cleanings with hot steam and subsequent filling with cold media, large stresses can be experienced in the membrane, which effect a permanent zero-point shift and thus a measurement error. This is attributable not the least to the fact that the transition region effects a relatively stiffer coupling between edge region and central region. The coupling should determine the particular temperature-dependent equilibrium position of the separating membrane.
The problem of CIP cleaning of membranes and the resulting hysteresis is discussed in U.S. Pat. No. 5,495,768. According to this, due to the momentum of the incoming cleaning medium, the pressure-transfer liquid is shifted under the separating membrane and this leads to a bulging of the planar membrane and permanent deformation of the joint between the membrane and the platform. It is doubtful whether this description of the causes of the permanent deformation is actually appropriate or complete, because temperature gradients are not sufficiently taken into consideration. In any event, this patent discloses, as its proposed solution, a pressure intermediary with a bowl-shaped separating membrane, which has a flat edge region for attachment of the separating membrane to a platform and a lightly waved, central region depressed relative to the edge region. Extending between the edge region and the central region is an inclined transition region, which connects the edge region and the central region together. The platform has a mainly congruent surface, and, thus also, a flat, annular edge region, a deeper, lightly waved, central region and an inclined annular transition region lying between the edge region and the central region. The edge region of the separating membrane is flushly connected by means of solder with the edge region of the surface of the platform. For unloading the edge region, the transition region is very stiff. I.e., in the case of a deflection of the separating membrane for accommodating a shifted oil volume, the transition region and the edge region protected by such are scarcely deflected. The deformation occurs exclusively in the central region of the separating membrane.
It need not be answered, whether the described approach achieves the desired unloading of the joint. To accommodate the oil volumes, there are required, in such case, nevertheless, such deflections of the separating membrane that a plastic deformation and, therewith, a zero-point shift can occur. The required large deflections in the central region come, not lastly, from the stiffness of the transition region, which limit a deflection of the volume-efficient zones of the central region, thus the annular zones with large radii.